It is well known that aircraft typically employ braking systems having brake disc stacks formed from a plurality interleaved discs. The discs are alternately splined to a wheel and torque tube such that alternating rotor and stator discs are present within the stack. A stationary end plate is provided at one end of the stack and is supported by a plurality of stationary fingers. An axially movable pressure plate is provided at the opposite end and is moved by a plurality of pressure pistons. The application of force to the pressure plate urges the stator and rotor discs into frictional engagement with each other, providing the necessary braking action for the aircraft.
Presently known aircraft brake discs are made of steel, carbon, or appropriate composites. Regardless of the type of material employed, tremendous heat is generated in the brake disc stack during a landing operation. It is well known that excessive heat within a vehicle braking system is generally harmful to the system. It is further well known that certain braking materials such as carbon and composites are desired for their light weight and thermal properties. However, such carbon and composite materials are quite expensive, far exceeding the cost of steel. Accordingly, there is a need when employing such materials to maximize material usage. In order to improve upon standard braking systems which have stators and rotors of equal thicknesses, U.S. Pat. Nos. 4,613,017 and 4,742,895 generally teach that thick rotors used in combination with thin stators or thin rotors in combination with thick stators, combined with an intermediate overhaul technique, can improve material (carbon) utilization. Furthermore, U.S. Pat. No. 5,295,560 teaches that a plurality of thick rotors and stators grouped in the middle of a brake stack with a plurality of thin rotors and stators grouped at each end of the brake stack will improve carbon utilization while more evenly distributing heat than in other such brake assemblies. By more quickly and evenly distributing heat throughout the braking system, aircraft are permitted to return to operation in a shorter period of time.
Generally, it is known that the braking system disclosed in the aforementioned U.S. Pat. No. 5,295,560 provides improved cooling characteristics for vehicle braking systems. Typically, brake discs begin to cool immediately following landing. However, heat is not dissipated as quickly as desired because braking operations incident to taxiing retard any cooling effect. Modern materials such as carbon and other composites are employed to withstand the high braking temperatures encountered, but safety considerations preclude use of the aircraft following landing for a sufficient period of time to allow the brake discs to cool below a predetermined temperature.
In the aircraft industry, each aircraft has a characteristic turnaround or ground waiting time. This is the time required for the hottest brake disc in the brake disc stack to cool below a specific temperature. For example, it is generally desired that the temperature of the hottest brake disc be below the ignition point of the hydraulic brake fluid prior to takeoff. Therefore, the turnaround time characteristic is a function of the aircraft landing speed, mass, brake system characteristics, taxiing procedures, and the like. As such, the more time it takes for the brakes to cool, the more costly it is for the airlines as the aircraft sit idle.
When using certain carbon or composite braking materials, it has been found that the brake discs may actually distort or axially deflect when force is effected between the pressure plate and end plate. Specifically, in the thermally balanced brake having thin rotors adjacent both the pressure plate and end plate, it has been found that such discs of certain materials achieve less than optimum braking efficiency as a result of such distortion or deflection precluding uniform forceful engagement of the disc interfaces. In order to maximize braking efficiency it is desirable to evenly distribute force or pressure across the disc interfaces throughout the stack while still allowing the stack to quickly cool to reduce aircraft turnaround times.